Low force wafer test probe with variable geometry

ABSTRACT

A system for testing functionality of die on a wafer including a plurality of contacts includes a support structure and a plurality of probes mounted to the support structure in an array. A configuration of each of the plurality of probes varies based on a position of the probe within the array to maintain uniform engagement between the plurality of probes and a corresponding plurality of contacts across the array.

DOMESTIC PRIORITY

This application is a continuation of U.S. application Ser. No.15/208,212, titled, “LOW FORCE WAFER TEST PROBE WITH VARIABLE GEOMETRY”,which was filed Jul. 12, 2016. The entire contents of U.S. applicationSer. No. 15/208,212 are incorporated by reference herein.

BACKGROUND

The present invention generally relates to testing wafers on whichelectronic circuits are formed, and more particularly, to a testconfiguration for testing an electronic circuit.

An important facet of the semiconductor industry resides in being ableto provide satisfactorily functioning semiconductor devices. Inparticular, such semiconductor devices may comprise wafers which aredivided into areas which form chips, the shapes and dimensions of whichare as close to identical as possible, so as to impart consistentuniform electrical properties thereto.

Generally, semiconductor devices on chips are ordinarily connected toeach other with thin strips of metal, referred to in the art asinterconnection metallurgy, which in turn contact the wafer surfacethrough a series of pads or bumps. Other connector pad configurationsmay include an array of electrical contacts or bumps which aredistributed over an area; for instance, the widely employed C4 bumps(controlled collapse chip connects). Such bumps or electrical contactsextend above the integrated circuits and have a generally spherical orround cross-sectional configuration.

Although wafers are formed as uniformly as possible through currentmanufacturing techniques, it is not always feasible that every chipproduced is perfect. In order to identify detective chips, electricaltests are performed to facilitate the sorting out of good chips andeliminating defective chips prior to the next step of manufacture.

Ordinarily, active testing of the wafers is performed by a test facilityin which the pads or areas on wafers possessing arrays of bumps, such asof C4 bumps, are contacted by an assembly incorporating test probes. Inorder to successfully probe the integrity of the pads or bumps, it isdesirable that an oxide layer, which inevitably forms on the surface ofthe C4 bumps, be ruptured and penetrated to ensure good electricalcontact with the probe while employing only a minimal force to inhibitdamaging the pads or bumps.

A substrate having a plurality of probes mounted thereto is used toperform a test on the plurality of C4 bumps of a wafer simultaneously.Each probe technology has a characteristic system compliance or springrate, thus the correct probe force occurs at a specific probedisplacement relative to the wafer. Consequently, current wafer testingpractice is to displace the wafer the specified distance into the probesystem. Unfortunately, the resulting forces may result in significantdeflection of the probe support structure. This may be especiallyproblematic for rigid probe arrays that incorporate a large number ofprobes because overdrive must be increased to overcome deflection of thesupport structure. As a result, the contact area, and therefore thecontact force, applied by the probes to each of the plurality of C4bumps may vary across the array.

SUMMARY

In one aspect, the present invention provides a system for testingfunctionality of die on a wafer including a plurality of contactsincludes a support structure and a plurality of probes mounted to thesupport structure in an array. A configuration of each of the pluralityof probes varies based on a position of the probe within the array tomaintain uniform engagement between the plurality of probes and acorresponding plurality of contacts across the array.

In one aspect, an embodiment of the present invention provides a systemfor testing functionality of die on a wafer including a plurality ofcontacts includes a support structure and a plurality of probes mountedto the support structure in an array. A configuration of each of theplurality of probes is selected based on an expected deflection of thesupport structure at a position associated with each of the plurality ofprobes.

In one aspect, an embodiment of the present invention provides a methodof forming a probe array includes determining an expected deflection ofa support structure of the probe array and connecting a plurality ofprobes to the support structure. A configuration of each of theplurality of probes is dependent on the expected deflection of thesupport structure at a position of the probe.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the advantages of the invention will be readilyunderstood, a more particular description of the invention brieflydescribed above will be rendered by reference to specific embodimentsthat are illustrated in the appended drawings. Understanding that thesedrawings depict only typical embodiments of the invention and are not,therefore, to be considered to be limiting of its scope, the inventionwill be described and explained with additional specificity and detailthrough the use of the accompanying drawings.

FIG. 1 is a side view of a system including an array of probes and awafer in an inactive position according to an embodiment;

FIG. 2 is a side view of a system including an array of probes and awafer in an active position according to an embodiment;

FIG. 3 is a side view of a system including an array of probes and awafer depicting bowing of the substrate under test load according to anembodiment;

FIG. 4 is a side view of a system including an array of probes havingvarying configurations according to an embodiment;

FIG. 5 is a side view of another system including an array of probeshaving varying configurations according to an embodiment;

FIG. 6. is a side view of yet another system including an array ofprobes having varying configurations according to an embodiment;

FIG. 7 is a side view of another system including an array of probeshaving varying configurations according to an embodiment; and

FIG. 8 is a cross-sectional view of two adjacent probes of an array anoffset in contact height to compensate for substrate deflection undertest load according to an embodiment.

DETAILED DESCRIPTION

Citation of “a specific embodiment” or a similar expression in thespecification means that specific features, structures, orcharacteristics described in the specific embodiments are included in atleast one specific embodiment of the present invention. Hence, thewording “in a specific embodiment” or a similar expression in thisspecification does not necessarily refer to the same specificembodiment.

Hereinafter, the present invention and various embodiments of thepresent invention will be described in more detail with reference to theaccompanying drawings. Nevertheless, it should be understood that thepresent invention could be modified by those skilled in the art inaccordance with the following description to achieve the excellentresults of the present invention. Therefore, the following descriptionshall be considered as a pervasive and explanatory disclosure related tothe present invention for those skilled in the art, not intended tolimit the claims of the present invention.

Referring now to FIG. 1, an example of an array 30 of test probes 32used to test the functionality of die on a semiconductor wafer 20 isillustrated. The semiconductor wafer 20 includes a plurality ofconductive bumps 22, also referred to as controlled collapse chipconnects (“C4 bumps”), which have a generally spherical or curved topshape. The semiconductor wafer 20 may include a base layer 24 formedfrom a substrate, such as a silicon material for example, having aspecific structure or openings. The base layer 24 may include a singleor multiple layers of material. The wafer 20 as supported in a testfixture may or may not have the capability of moving in one or moredirections prior to electrical contact for indexing the position of thewafer 20 and the circuit on the wafer 20 to be tested.

As shown, the probes 32 of the array 30 are mounted to a supportstructure 34 in a configuration such that each probe 32 is substantiallyaligned with one of the C4 bumps 22 on a semiconductor wafer 20. Each ofthe probes 32 has a longitudinal axis X which passes through the centerof the probe 32 such that a height of each probe 32 may be measuredalong the longitudinal axis. The longitudinal axis X of the probes 32 isgenerally aligned with a longitudinal axis Y (FIG. 2) of each of the C4bumps 22. When the probe 32 is in contact with C4 bumps 22, the distancebetween the structure 34 and the C4 bumps 22 is less than the length ofthe probes 32, to ensure that a pressure or force is applied to each ofthe C4 bumps 22. The pressure or force provides penetration of the C4bumps 22 resulting in piercing and exposing a new clean contact surfacefree of oxide below the former surface 26 of the C4 bump 22.

Typically each of the plurality of probes 32 arranged within aconventional array 30 is substantially identical. As the supportstructure 34, and therefore the probes 32, is moved into contact withthe C4 bumps 22, the support structure 34 tends to deflect or bow, asshown in FIG. 3. This deflection, which occurs based on how the supportstructure 34 is supported relative to the test fixture, the force perprobe, and the number of probes 32, causes the contact area between eachprobe 32 and a corresponding C4 bump 22, and the contact force appliedthereto, to vary across the array 30. As shown in the FIG., the probes32 arranged at the ends or exterior of the array 30 have increasedcontact with the corresponding C4 bumps 22, and therefore apply anincreased force thereto.

To compensate for deflection of the support structure 34 that occursunder a test load, whether measured or modeled deflection, theconfiguration of one or more of the probes 32 within an array 30 isvaried. In an embodiment, the configuration of one or more probes 32 ofthe array 30 is selected such that uniformity of one or more of thecontact area, displaced solder volume, and final force applied by eachprobe 32 is achieved at the interface between each probe 32 and acorresponding C4 bump 22, regardless of the location within the array30.

This customization of the probes 32 to achieve one or more uniformproperties across the array 30 may include altering one or more elementsof the probe geometry. For example, the height of the probe 32, andtherefore the height at which the probe 32 contacts a corresponding C4bump 22 can be varied across the array. In the illustrated, non-limitingembodiment of FIG. 4, the height of the pedestal 36 of one or moreprobes 32 and/or the height of the feature 38 extending from thepedestal 36 configured to contact the C4 bump 22 may vary across thearray, so that plurality of probes 32 contacts the spherical surfaces ofan array of C4 bumps 22 at different heights.

Alternatively or in addition, at least one of the size (surface area)and shape of the surface 40 of the feature 38 configured to contact a C4bump 22 may vary between adjacent probes 32 within an array 30. In anembodiment, this variation may be achieved by using a plurality ofprobes 32 having similarly shaped, but different size features 38. Forexample, as shown in FIG. 5, each of the plurality of probes 32 includesa generally frusto-conical or pyramid shaped feature 38. However, thecross-sectional area of at least one of the plurality of features 38within a horizontal plane is different than the remainder of theplurality of features 38. As a result, the surface area of the features38 configured to contact and engage an adjacent C4 bump 22 may vary.

In another embodiment, the type of probes 32, and therefore the type offeatures 38 for contacting the plurality of C4 bumps 22, within thearray 30 may vary. For example, the array 30 illustrated in FIG. 6includes a first probe 32 including a generally cylindrical feature 38having a generally constant cross-sectional area over its height and asecond probe 32 including a generally frusto-conical or pyramid-shapedfeature 38. As a result, the size and/or shape of the surface 40 of thefeature 38 configured to contact a corresponding C4 bump 22 is differentbetween the probes 32. It should be understood that the probeconfigurations illustrated and described herein are intended as anexample only, and a probe 32 having a feature 38 of any configuration iswithin the scope of the disclosure.

In another embodiment, as illustrated in FIGS. 7 and 8, one or more ofthe probes 32 may include a cavity 42 within which one or more features44 configured to contact the exterior surface 26 of the C4 bump 22 arelocated. In the illustrated, non-limiting embodiment, the features 44include blades that extend generally inwardly from the periphery of thecavity 42 towards a central axis thereof. In an embodiment, the diameterof the cavity 42 of each probe 32 may vary such that each probe wouldcontact a corresponding spherical surface of the C4 bump 22 at adifferent height. The variation in diameter could be selected to achievea predetermined offset, illustrated schematically at H in FIG. 8, in thecontact positions. It should also be understood that the variations inprobe configuration within an array 30 illustrated and described hereinare intended as an example only and that variations are also within thescope of the disclosure.

By designing an array 30 such that each probe 32 has a specificconfiguration dependent on the position of the probe 32 within the array30 and an expected or known deflection of the support structure 34 tooccur at that location, the contact that occurs between each probe 32and a corresponding C4 bump 22, and therefore the load that is appliedto each C4 bump 22 may be optimized. That is to say, uniformity can beachieved across the array, or certain probes could be targeted toprovide increased or decreased contact relative to others.

The foregoing detailed description of the embodiments is used to furtherclearly describe the features and spirit of the present invention. Theforegoing description for each embodiment is not intended to limit thescope of the present invention. All kinds of modifications made to theforegoing embodiments and equivalent arrangements should fall within theprotected scope of the present invention. Hence, the scope of thepresent invention should be explained most widely according to theclaims described thereafter in connection with the detailed description,and should cover all the possibly equivalent variations and equivalentarrangements.

The invention claimed is:
 1. A system for testing functionality of dieon a wafer including contacts, comprising: a support structure; probesmounted to the support structure in an array, wherein a configuration ofeach probe varies in shape or dimension from a neighboring probe basedon a position thereof within the array to maintain uniform engagementbetween the probes and a corresponding contact across the array, whereineach probe has an axis along which a probe length is measurable andwhich is aligned with an axis of a corresponding contact and, with eachprobe contacting the corresponding contact, a distance between thesupport structure and the contact is less than each probe length.
 2. Thesystem according to claim 1, wherein the configuration of each probe isdependent on a deflection of the support structure, which is measured ormodeled under a test load.
 3. The system according to claim 1, wherein acontact area between each probe and the corresponding contact issubstantially uniform across the array.
 4. The system according to claim1, wherein a displaced solder volume between each probe and thecorresponding contact is substantially uniform across the array.
 5. Thesystem according to claim 1, wherein a contact force applied by eachprobe to the corresponding contact is substantially uniform across thearray.
 6. The system according to claim 1, wherein a configuration of atleast one probe within the array is different due to variance in shapeor dimension from a remainder of the probes.
 7. The system according toclaim 6, wherein a geometry of the at least one probe within the arrayis different from the remainder of the probes.
 8. The system accordingto claim 7, wherein an overall height of the at least one probe withinthe array is different from the remainder of the probes.
 9. The systemaccording to claim 7, wherein a shape of the at least one probe withinthe array is different from the remainder of the probes.
 10. The systemaccording to claim 7, wherein a surface area of a contact surface of theat least one probe within the array is different from the remainder ofthe probes.
 11. A system for testing functionality of die on a waferincluding contacts, comprising: a support structure; probes mounted tothe support structure in an array, wherein a configuration of eachprobes varies in shape or dimension from a neighboring probe and isselected based on a deflection of the support structure at a positionassociated therewith, wherein each probe has an axis along which a probelength is measurable and which is aligned with an axis of acorresponding contact and, with each probe contacting the correspondingcontact, a distance between the support structure and the contact isless than each probe length.
 12. The system according to claim 11,wherein each probe is configured to engage the corresponding contactsand a contact area between each probe and the corresponding contact issubstantially uniform across the array.
 13. The system according toclaim 11, wherein each probe is configured to engage the correspondingcontacts and a displaced solder volume between each probe and thecorresponding contact is substantially uniform across the array.
 14. Thesystem according to claim 11, wherein each probes is configured toengage the corresponding contacts and a contact force applied by eachprobe to the corresponding contact is substantially uniform across thearray.
 15. The system according to claim 11, wherein a configuration ofat least one probe is different from a remainder of the probes.
 16. Amethod of forming a probe array comprising: measuring or modelingsupport structure deflection of a support structure under a test load;determining an expected value of the support structure deflection fromthe measuring or modeling; and connecting probes to the supportstructure, wherein a configuration of each probe varies in shape ordimension from a neighboring probe and is dependent on the expectedvalue of the support structure deflection at a position thereof.
 17. Themethod according to claim 16, wherein a configuration of at least oneprobe is different from a remainder of the probes.
 18. The methodaccording to claim 17, wherein a geometry of the at least one probe isdifferent from the remainder of the probes.
 19. The method according toclaim 18, wherein an overall height of the at least one probe isdifferent from the remainder of the probes.
 20. The method according toclaim 18, wherein a shape of the at least one probe is different fromthe remainder of the probes.
 21. The method according to claim 18,wherein a surface area of a contact surface of the at least one probe isdifferent from the remainder of the probes.